Fabric load lifting sling and similar flexible tension member having increased ability to withstand shock loads



R. NORTON 3,466,080

2 Sheets-Sheet 1 FABRIC LOAD LIFTING SLING AND SIMILAR FLEXIBLE TENSION MEMBER HAVING INCREASED ABILITY TO WITHSTAND SHOCK LOADS I I I II II. u II l I| I I I I I I I l l I I I I I l I I I I I I I I I CONVENTIONAL Sept. 9, 1969 Filed oct.

I n I "I IIILIIIIQNUIIIHIIIJM" Illu I I Sept. 9, 1969 R. NORTON FABRIC LOAD LIFTING SLING AND SIMILAR FLEXIBLE TENSION MEMBER HAVING INCREASED ABILITY TO WITHSTAND SHOCK LOADS Filed oct. s. 1967 2 Sheets-Sheet 2 United States Patent O 3,466,080 FABRIC LOAD LIFTING SLING AND SIMILAR FLEXIBLE TENSION MEMBER HAVING IN- CREASED ABILITY T WITHSTAND SHOCK LOADS Roy Norton, Milwaukee, Wis., assignor to The Wear-Flex Corporation, Milwaukee, Wis., a corporation of Wisconsin t t Filed Oct. 5, 1967, Ser. No. 674,057 Int. Cl. B66c 1/12 U.S. Cl. 294-74 ABSTRACT 0F DISCLOSURE A load lifting sling made of fabric webbing, parts of which are joined together in lengthwise overlapping relation by stitching which is confined to portions of the lap that are separated from one another along the length of the lap and are of substantially the same area. The medial unsecured portions of the overlapping parts connecting the separated stitched areas are of unequal length so that tension loads on the sling tending to tear the stitched joints apart are successively transmitted to the stitched portions, with the result that the stresses concen- `trated at the ends of the stitched portions are reduced and the ability of the sling to withstand shock loads is increased.

This invention relates to fabric load lifting slings and similar eXible tension members.

The present invention is closely related to that of my copending application Ser. No. 674,060, led concurrently herewith. Thus, while this invention has as one of its objects the provision of a load lifting sling capable of withstanding high shock loads, the manner in which that objective is achieved also accomplishes thepurpose of the invention of my other application, which is to provide a load lifting sling with an indicator that enables an operator to see if a load can be safely lifted with the sling before the load is even lifted off the floor.

It is also an object of this invention to increase the strength of a stitched lap joining lengths of fabric webbing, beyond that which can be gained by increasing the With these observations and objects in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings. This disclosure is intended merely to exemplify the invention. The invention is not limited 16 Claims to the particular structure desclosed, and changes can be made therein without departing from the invention.

The drawings illustrate several complete examples of the physical embodiments of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIGURE l is` a perspective view of a load lifting sling of the endless type made of fabric webbing and having a lapped joint embodying this invention;

FIGURE 2 is a perspective view on an enlarged scale of the lapped joint by which the ends of the webbing are secured together to produce the endless sling shown in FIGURE 1; p

-FIGURE 3 is a perspective view similar to FIGURE 2, showing the conventional lapped joint heretofore used to secure overlapping portions of fabric webbing together;

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FIGURE 4 is a perspective view of a stitched lap ernbodying only one of the two aspects of this invention;

FIGURE 5 is a perspective view of part of a fabric tension member made as a woven entity, and embodying the other aspect of this invention;

FIGURE 6. is a perspective view similar to FIGURE 5, illustrating a modification of the woven tension member; and

FIGURE 7 is a perspective view of a flexible tension member similar to that shown in FIGURE 5, but consisting in this case of several plies of webbing that are stitched together.

Referring to the accompanying drawings, the numeral 8 designates generally a load lifting sling of the endless type, made of fabric webbing, which is usually nylon. Being endless, the sling 8 has the end portions of the length of webbing of which it is made atwise overlapping and stitched together to form `a stitched joint or lap 9.

Prior to this invention, the stitched laps of fabric load lifting slings-and similar elongated flexible tension members-were always made as shown in FIGURE 3. Throughout the entire area of the lap, the overlying portions of the webbing were stitched together, the stitching 10 usually extending back and forth ,lengthwise of the lap and covering an area depending upon the si'ze of the webbing and the rated capacity of the sling.

From years of experience, it was learned that the stitched laps of fabric slings, and more particularly the stitching thereof, was the limiting factor in constructing slings of higher capacities. While increasing the length of the lap satisfied the strength requirements of medium strength webbing, it did not do so in the higher strength webbing. For instance, increasing lap length in webbing of 10,000 pounds (ten thousand pounds) per inch of webbing width did not appreciably increase the lap strength above (eighty percent) of the web breaking strength. The stitching was thus the weakest part of the sling and invariably if a sling failed, it was the stitches that broke. First lthe stitches at the ends of the lap tore and, with that, all the remaining stitches would go Undoubtedly the reason for the failure of the endmost stitches lies in the concentration of stress at these points, which is greatly aggravated by the high stretch factor of nylon webbing. It can elongate as much as one-third it original unstretched length before it breaks.,`

I have discovered that dividing the stitch'd lap into shorter lengths spaced from` one another lengthwise of the lap results in a stronger lap. In fact, the stitched lap is stronger than the webbing.

I have also found that by having the medial portion of one of the plies of a stitched lap longer than the other plies so that it has slack, improves the ability of the sling to absorb shock loads and in addition this difference in length between the unsecured medial portions of the two plies provides an indicator =by which an `operatior can readily see if the load on the sling exceedits rated capacity.

Accordingly, in the preferred embodiment of the in vention, best seen in FIGURE 2, the lap 9 has two stitched portions 11 and 12 spaced from one another lengthwise of the lap by an unstitched medial portion 13 wherein one of the two overlying pliesf-in this case the top ply 14-is longer than the bottom ply 15 when the latter is in its free unstretched condition. This produces the outwardly displaced or bowed formation illustrated, though it should be understood that this shape has no significance. It is only the slack in the ply 14 which results from the difference in length of the two ply portions 14 and 15 that is important.

Because of this slack, shock loads to which the sling may be subjected are less likely to break the sling and particularly its sewed joint. This advantage no doubt stems from the fact that the time interval involved in stretching the medial portion to take up the slack in the portion 14 decelerates the force producing the shock load.

This shock absorbing aspect of the invention is of course not limited to load lifting slings in which the provision of the slack is an incident of the stitched lap, but has a far more general utility. Thus, as shown in FIGURE 5, nylon webbing can be woven with multi-ply segments 16 consisting of separate long and short plies 17 and 18, respectively, spaced apart lengthwise of the webbing and connected by segments 19 that may be considered as integrated, in contrast to the multi-ply segments 16. The cross sectional area of the integrated sections 19 is equal to the sum of the areas of the plies 17 and 18, and because of the difference in length of the plies 17 and 18, the webbing has built-in shock absorbers that very effectively and significantly increase the capability of the webbing to handle shock or impact loads.

While the two-ply shock absorbers of FIGURES 2 and 5 increase the capability of a tension member to withstand shock loads, even greater improvement in this regard can be realized by adding a third ply 20, as shown in FIGURES 6 and 7. This third ply is longer than the slack ply 17 so that before the third ply assumes any part of the shock load on the tension member, the ply 17 must be stretched as well as the middle ply 18. This results in a two-stage deceleration of the force which produces the shock load.

As in the case of the woven tension member of FIG- URE 5, the three-ply segments are spaced apart length- Wise of the tension member and are connected by integrated segments 19, the cross sectional area of which equals the sum of the cross sectional areas of the three plies 17, 18 and 20, whether the tension member is one woven entity, as in FIGURE 6, or is made of three separate lengths of webbing that are stitched together, as in FIGURE 7.

The stitched construction of FIGURE 7 does not have the elasticity of the woven construction since the stitching 21 which secures the three plies together at each of the integrated segments 19 of the tension member, resists stretching of these integrated segments. Hence, for some purposes, the stitched construction may not be as satisfactory as the more elastic woven construction.

As shown in FIGURE 7, the stitching 21 which secured the plies 17 and 18 together extends farther than the stitching securing the ply to the other two plies.

As noted hereinbefore, I have found that a stitched lap joining two lengths or sections of webbing as in the load lifting sling of FIGURE l, is stronger if the stitching is confined to separate areas that are spaced lengthwise of the lap. This division of the stitched lap into separated areas may be obtained in the manner shown in FIGURE 2, wherein the stitched portions 11 and 12 are connected by long and short medial portions to provide the slack that makes possible the increased capability to withstand shock loads, as explained.

Division of the stitched lap into spaced apart areas to increase the strength of the lap can also be obtained in the manner shown in FIGURE 4, wherein the two plies 13 and 14 are of the same length.

The explanation for the increase in strength in the stitched lap brought about by the substitution of two spaced apart stitched areas for one continuous stitched area equal in `size to the sum of the spaced areas, no doubt resides in the fact that with one single large stitched area (the conventional way of making a lap) the stress upon the stitching which is concentrated at the two ends of the lap is substantially greater than the stress which exists at the four ends of the two separated, but shorter, areas of stitching.

From the standpoint of only increasing the strength of the stitched lap, the construction shown in FIGURE 4 is preferable to that of FIGURE 2 since, in the former, the stress resulting from tension loads on the lap is irnmediately concentarted at the four points a, b, c and d, while in the structure shown in FIGURE 2, the stress is initially concentrated at only the two points a and b, and the second pair of stress concentration points c and d do not exist until the slack in the medial portion 14 is taken up. However, the construction of FIGURE 2 has the advantage of combining increased capability to withstand shock loads with increased strength in the stitched lap.

To illustrate the extent of the increase in strength of the stitched lap of this invention, two load lifting slingsidentical in every respect except for their stitched lapswere subjected to a laboratory breaking test. One of these test slings had the conventional lap, as shown in FIGURE 3; in the other, the lap was like that of FIGURE 2. The area of the conventional stitched lap was equal to the sum of the areas Vof the two separated stitched lap areas. By means of an Olsen tensile machine, a steady pull of increasing magnitude was applied to the test slings. The sling with the conventional lap failed at 25,000 pounds.` The stitches tore and the lap pulled apart. The other sling, in which the stitched lap was like that of FIGURE 2, failed at 30,000 pounds, but not in the lap. The Webbing tore at a point spaced from the lap.

The difference in length of the overlying plies in each of the several embodiments of the invention that have this feature, and especially those illustrated in FIG- URES 1, 2 and 5, in addition to improving the ability of the sling to withstand shock loads, also provides a load indicator by which an operator can tell if the load on the sling exceeds its rated capacity, even before the load is lifted off the floor. If the slack in the longer ply is fully taken up and it comes in contact for its full length with the other ply, the operator is apprised that the sling is being overloaded.

Nylon webbing from which the slings are customarily made, as is well known, is available with a calibrated stretch factor that identifies the amount the webbing will elongate before it ruptures, and since-as is also well known-the elongation is uniformly proportional with increasing tension, it is evident that the selection of the proper differential in length of the overlying plies to have the initial surface-to-surface contact thereof occur when the load on the sling is at its rated capacity, or any selected fraction thereof, is but a matter of arithmetic.

From the foregoing description taken with the accompanying drawings, it will be apparent that this invention has significantly enhanced the capability of fabric load lifting slings and flexible tension members of general utility, to withstand shock loads, and that it also increases the strength of the stitched laps of fabric load lifting slings.

What is claimed as my invention is:

1. An elongated flexible tension member suitable for use as a load lifting sling, said tension member being made of material having an appreciable stretch factor, and being characterized in that for at least a part of its length the tension member consists of (a) a plurality of separate elongated tension elements,

the ends of the adjacent ones of which are in juxtaposition to one another, and v (b) endwise spaced integrated portions to which the ends of said separate elements are joined,

said separate elements being of unequal length so that when free and unstretched the shorter element draws the ends of the longer element towards one another and forms slack in the longer element which must be taken up before the longer element can carry any part of a tension load applied to the tension member.

2. The flexible tension member of claim 1, wherein said separate elongated elements `and the endwise spaced integrated portions to which the ends of said elements are integrally joined are one Woven fabric entity.

3. The flexible tension member of claim 1, wherein said separate elongated elements are lengths of fabric webbing yand said integrated portions are extensions of said lengths of webbing securely stitched together.

4. The elongated flexible tension member of claim 1, wherein there are only tow separate elongated elements.

5. The elongated flexible tension member of claim 1, wherein there are more than two separate elongated elements, no two of which are of the same length when the tension member is unstretched.

6. A exible tension member suitable for use as a load lifting sling, comprising:

(A) a plurality of band-like elements of material having an appreciable stretch factor,

said elements being in atwise overlying relation;

and

(B) means securing said elements to one another in at surface-to-surface engagement at spaced intervals along the length of the tension member,

the unsecured portion of one of said band-like elements being shorter than its adjacent element, so that when the tension member is in its free unstretched condition the unsecured portion of said adjacent element is slack and incapable of carrying any part of a tension load on the tension member until the shorter unsecured portion has been stretched suiciently to take up said slack.

7. A flexible tension member comprising:

a length of fabric webbing having an appreciable stretch factor,

said webbing having at least one part of its total length comprised of multiple plies connected only at the ends thereof, one of said plies being shorter than the rest,

so that when said shorter ply is in its free unstretched condition every other one of the rest of the plies is slack and incapable of sharing tension loads applied to the tension member until the shorter ply has been stretched sui-ciently to take up said slack.

8. The exible tension member of claim 7, wherein the entire length of fabric webbing including said multiple ply part thereof is one woven entity.

9. A load lifting sling of fabric webbing having a lap formed by superimposed lengths of webbing stitched together, characterized in that:

(A) the stitching securing said superimposed lengths of webbing together is confined to separated portions of the lap spaced apart lengthwise of the lap,

said stitched portions being of substantially the same area so that all stitched portions have the same strength; and

(B) the superimposed portions of the lap connecting the separated stitched portions being of unequal length.

10. Means for indicating when a band of stretchable material has a predetermined tension force applied to it comprising:

a flexible strap overlying said band; and tension imparting connection means securing the ends of the strap to the band, so that tension on the band can be applied to the strap whereby the latter can share such tension forces, the length of the strap being greater than the length of the part of the band it overlies when the latter is in its free unstretched condition, but being equal to the length of the underlying part of the band when the latter has been stretched the amount it elongates upon being subjected to said predetermined tension force and said strap is placed in tension,

whereby initial contact of the strap for its entire length with the underlying part of the band indicates that the predetermined tension force has been applied to the band.

11. The structure of claim 10, wherein said band of stretchable material constitutes part of a load lifting sling,

and wherein said predetermined tension force corresponds to the rated capacity of the sling,

so that initial contact of said strap for its entire length with the underlying part of the band indicates that the load on the sling is equal to or greater than the rated capacity of the sling.

12. The structure of claim 11, wherein said band is a length of fabric webbing.

13. The structure of claim 11, wherein said band is a length of nylon webbing, and wherein said band of nylon webbing constitutes the entire sling.

14. The structure of claim 13, wherein said strap is an end portion of said length of nylon webbing.

1S. A load lifting sling consisting essentially of a fabric band which has predetermined elongation per pound of load lifted by the sling, parts of saidl band being lapped and stitched together to form a loop, said sling being characterized in that:

one of said lapped parts has a medial slack section wholly detached from said other part, and end sections which are stitched to said other part,

the medial slack section being longer than the portion of said other part which it overlies when the latter is in its free unstretched condition but substantially equal to the length thereof when said other part has been stretched by a load on the sling of predetermined weight,

so that initial contact of the medial slack section for its entire length with said other part indicates that the load on the sling is of said predetermined weight or greater.

16. Means for indicating when a tension load on a stretchable fabric band reaches a predetermined magnitude, comprising:

(A) an indicator member;

(B) irst connection means iixed to the band and to one end of said indicator member to secure said end of the indicator .member to the band; and

(C) second connection means xed to the band at a point spaced lengthwise from said rst connection means and connected with the indicator member, both of said connection means being capable of transmitting tension forces which are applied to the band, onto the indicator member so that the indicator member is placed in tension when the portion 0f the band between said two yconnection means has been stretched a distance proportional to the elongation of the band under a tension load of said magnitude,

whereby initial application of tension on the indicator member provides the desired indication.

References Cited UNITED STATES PATENTS 2,387,737 10/ 1945 Black 73-443 3,290,083 12/1966 Norton 294-74 FOREIGN PATENTS 1,224,354 6/ 1960 France.

ANDRES H. NIELSEN, Primary Examiner U.S. Cl. X.R. 73--143 

